Hey:)

If you got a seat in the first phase and went for verification,you don't need to worry about the stamp getting cancelled manually. If you had chosen the upgrade option,then in the second phase if you get a new college,your allotment and stamp are automatically cancelled and you just move to the new one .But if you had frozen the seat(no upgrade),then you can't change it in the 2nd round unless you formally withdraw your admission,and once you withdraw your admission you won't be considered further. So, basically,the system itself handles the cancellation if you go for upgradation.